This invention has particular utility in an active clearance control system of the type exemplified by U.S. Pat. No. 4,069,662 granted to Redinger et al on Jan. 24, 1978 and assigned to the same assignee as this patent application. As disclosed in that U.S. Patent, air from a source, such as the fan discharge air, is fed to a plurality of spray bars that direct the air to impinge air on the exterior of the engine case in proximity to the turbine. This serves to shrink the case at a predetermined time relative to the engine's flight envelope for the purpose of maintaining a close gap between the outer periphery of the turbine buckets and its associated outer air seal that surrounds the same.
The clearance control system currently being employed utilizes an on-off type of valve. I have found that I can replace these existing valves with my invention, obtain flow modulation and yet achieve a highly reliable valve that is characterized as being relatively simple. In accordance with my invention the valve is opened to some discrete position in response to a control input command signal. Each discrete position can be tailored to give a given amount of airflow for any given application without incurring any complicated redesign or major changes in the valve structure.